This invention relates to dynamic pressure and force transducers, and more particularly to a novel piezoelectric transducer for measuring dynamic pressure in tubes or passages without introducing discontinuities in the surfaces thereof that could appreciably disturb the flow process.
One area of use of the present invention is in measuring through the case wall of an unmodified cartridge the chamber pressure developed during tests of guns and ammunition. One form of prior art apparatus for measuring pressure in the testing of guns and ammunition included a piston and cylinder mechanism actuated by chamber pressure to deform a lead or copper disk and appropriately is known as a lead or copper crusher. The resulting deformation of the disk is measured with a micrometer to give relatively limited information concerning peak pressure or the time integral of pressure. Although this apparatus presently remains in use and provides industry standards, it is not really adaptable to automated testing, process analysis, or testing of automatic weapons. Furthermore, in order to deform a lead or copper disk the piston must move an appreciable distance, and this in turn requires that pressure be applied to the piston face from a hole or aperture drilled in the cartridge case and aligned with the piston face.
Although the merits and potential of quartz piezoelectric instruments for ammunition testing were recognized many years ago, the difficulties arising from the need to condition the ultrahigh impedance signal delayed any significant progress until relatively recent times. A long history of evolutionary development, however, indicates that quartz is perhaps the only electronic transduction mechanism with the ruggedness, rigidity, stability and durability required in automated ammunition testing. Early versions resembled the crusher mechanisms with the lead or copper disks replaced by a quartz piezoelectric element. Separate piston and cylinder adaptors transmitted chamber pressure into a force acting on conventional quartz pressure transducers. However, lubrication and sealing of the piston presented serious operational problems, and the inertia of the piston introduced spurious dynamic effects which tended to obscure the pressure signal.
Another version was the high pressure, diaphragm type wherein a thin sheet metal membrane secured to the face of the instrument sealed against pressure and served as a flexure to mechanically isolate the sensitive inner quartz column from stresses acting on the outer housing. One problem with this version is that after a relatively few tests the diaphragm is subject to fatigue or change in effective area thereby impairing the sensitivity. Another problem is that this version requires a recessed mounting with connecting passage or cavity, thermal insulation of the diaphragm, and access hole in the cartridge wall or mounting downstream of the projectile, all of which have been found to introduce spurious signals and to sometimes interfere with the projectile motion.
In an attempt to overcome these problems, the machined diaphragm transducer was developed and is characterized by an essentially one piece or integral housing and machined flexure. This prolonged instrument life somewhat but not by a considerable amount, and the difficulties in modifying cartridges, maintaining connecting passages, and replacing thermal insulation restricted widespread acceptance and use of this version. Although mounting of a conventional diaphragm type transducer in a slight recess downstream of the case mouth avoids the necessity of modifying the cartridge, it does not measure true chamber pressure because of the throttling effects at the cartridge mouth and gas resonances in the connecting passages. Measured results are observed to differ appreciably in peak amplitude and waveform.